Occasionally a na´ve newbie wanders in and tries to explain to atheists that Satanists do not worship Satan. S\he gets run out of town.

I've seen a number of these militant atheists explicitly state that they wish to see all religion eliminated. To them, this includes the non-theistic religions as well. These are the types of atheists who, even when you correct their ignorant assumptions of what Satanism is, will just try to find some other aspect of Satanism to thoroughly criticize.

As for American Atheists, I don't see anything in their mission statement that would inherently contradict the principles of Satanism:http://atheists.org/content/aims-and-principles(Though personally I find their definition of "atheism" to be absurdly detailed. "Atheism" really doesn't mean anything more than not believing in a deity.)